This invention relates to gain tuning in radio frequency circuits. In particular, the present invention relates to improvements in gain tuning across wide frequency bands.
A radio receiver will typically be configured to receive radio frequency signals across a range of frequencies, with frequency tuning circuitry being employed to allow the radio receiver to tune into a signal at a particular frequency within the range. Generally a range of frequencies is termed a band, particularly if the frequencies of the band are used for a common purpose or type of radio signal. With conventional tuning circuits, it is difficult to achieve constant gain across a frequency band, especially when the band is wide. The gain of a conventional tuning circuit therefore varies with the frequency to which the receive circuitry is tuned.
The variation in gain of a tuning circuit has a negative effect on the performance of a radio receiver because the following amplification and signal processing stages must be configured to take the gain variation into account (e.g. to avoid clipping of the signal). This leads to decreased receiver efficiency and poor performance in some regions of the frequency band.
Previously, the problem of gain “ripples” in the tuning circuits of radio receivers has been approached through the use of additional active circuitry (that consumes additional power), which can improve the gain flatness of a tuning circuit but which introduces other problems. Furthermore, the prior art methods for dealing with gain variation do not cope well with very wide frequency bands, such as those used in ultra-wideband (UWB) communications. Conventional UWB radio receivers and other large bandwidth amplifier therefore often employ multiple tuning circuits in order to cover each band group of the UWB spectrum. Furthermore, such receivers and amplifiers generally utilise negative feedback to cope with the large bandwidth, which lowers the gain and increases the noise in the received/amplified signal.
The variation in gain with frequency of a tuning circuit in a typical wide bandwidth receiver is shown in FIG. 1. The figure shows the gain of the tuning circuit dropping off towards the edges 11 of the frequency band of interest, Δf. As the bandwidth over which the tuning circuit operates is increased, this characteristic becomes increasingly severe. The 3 dB bandwidth of the tuning circuit is given by f0 (the mean of f1 and f2) divided by 2Q (where Q is the equivalent quality factor of the tuning circuit). To increase the 3 dB bandwidth of the circuit, one therefore has to lower the quality factor, Q. This can help to flatten the gain variation over the frequency band but in order to maintain a given gain level, the current consumption of the tuning circuit increases (for a given gain the bias current of the tuning circuit is inversely proportional to Q).
Current bleeding has been used with some success to adjust the gain of radio tuning circuits. As shown in FIG. 2, current bleeding provides a predetermined bypass current across the tuning circuitry (shown as a simple LC circuit) which helps to flatten the gain of the tuning circuit. In FIG. 2, current source 22 provides the bleed current Ibleed across the tuning circuit comprising inductor 21 and capacitor 23. However, current bleeding as a gain adjust technique increases the noise level of the signals passed onto subsequent stages of the radio receiver and does not easily allow fine gain adjustments. Current bleeding is only practical to implement for large gain steps (of the order of 10 dB).
Conventional gain adjustment techniques suffer from several problems and do not enable fine control of the gain of a circuit across a wide frequency band without introducing noise, additional parasitic capacitances, or affecting the input impedance of the circuit. There is therefore a need for an improved tuneable circuit that does not suffer from these problems and allows fine control of the gain across a wide frequency band.